In motor vehicle air conditioning systems, there have been various approaches to detecting when the refrigerant becomes too low for continued system operation and either signaling an alert or shutting the system off. For example, one approach, for cycling clutcn systems only, has been to measure the temperature change in the refrigerant at the point of entering the evaporator for a brief time on start up. This is accomplished with a thermistor with a large change in temperature indicating a low charge. Another means of detecting low charge has been to employ a temperature switch that senses compressor discharge temperature and shuts the compressor off or signals an alert when the temperature exceeds a certain high or abnormal temperature. Another way of detecting a low charge has been to sense the liquid level of the refrigerant in the receiver or accumulator as the case may be. This is accomplished with a float switch that is operated by the refrigerant liquid level diminishing to a certain low or abnormal level.
Another method of low refrigerant charge detection has been to use a suction superheat switch which senses suction pressure and suction gas temperature and indicates low charge when the super heat of the suction gas of the compressor reaches a certain value. This superheat switch was not biased or compensated by ambient temperature. Another way of detecting low refrigerant charge has been to use a pressure switch which senses compressor discharge pressure. This switch indicates low charge when the pressure switch senses low discharge pressure.